PML is a subacute, fatal demyelinating disease of the Central Nervous System (CNS) that affects individuals with underlying defects in immunity. This once considered rare disease is more frequently seen after the AIDS pandemic. The significantly higher incidence of PML in patients with AIDS than in any other immunosuppressive condition suggests that there may be an interaction between HIV-1 and JC Virus. Previous in vitro studies have demonstrated direct intercommunication between HIV-1 and JCV through the HIV-1 encoded regulatory protein Tat. This interaction may also be mediated by cytokines and transcription factors. Although there is no strong evidence for the infection of oligodendrocytes by HIV- 1, it is postulated that soluble Tat released from the HIV-l infected cells in the CNS such as microglia, can be taken up by the neighboring uninfected cells, including oligodendrocytes harboring the opportunistic JCV. Activation of JCV could also be mediated indirectly through enhanced expression and release of cytokines such as TNF-alpha and TGF-beta from the HIV-1 infected cells. TNF-alpha can induce expression and activity of inducible transcription factors, such as NFkB which increases expression of the JCV genome, via the KB motif positioned in the viral promoter region. On the other hand, TGF-beta, through the SMAD pathway, could also lead to activation and elevated expression of the JCV genome. With this notion, we hypothesize that activation of JCV by Tat could be mediated by localization of Tat in the nucleus of JCV infected oligodendrocytes, and indirectly by altering expression of cytokines, which leads to enhanced activity of NFKB subunits p50 and p65, and TGF-beta /SMAD in the JCV infected oligodendrocytes. We will perform cell culture studies to investigate the regulatory pathways, including TNF-a and TGF-beta that upregulate JCV replication and we will also carry out immunohistochemical studies in human AIDS and non- AIDS related PML autopsy specimens to illustrate replication of JCV in oligodendrocytes at the level of NFKB and SMADs, as well as Tat in cells which are infected with JCV. The results from these experiments will provide information to elucidate the role of HIV-1 and its transactivator protein Tat in the activation of JC Virus, through direct and/or indirect mechanisms, and therefore in the development of PML in patients with AIDS.